Is time travel impossible?

The idea that we can travel in time should make sense. We can travel in space from one co-ordinate to another so a similar thing should be possible with time; to move from one moment in time to another. It sounds simple, but time travel really doesn’t make sense! Going into the past and living the same moment again, or going into the future and missing out a whole chunk of what would otherwise have been part of your life doesn’t seem logical. “What’s happened has happened, and the future hasn’t yet been decided.” But is this old wisdom true? Or is the inevitability of predestination, “Que sera sera” (“whatever will be will be”) more likely?

Then there are paradoxes associated with time travel; the most famous is the “Grandfather paradox” where a time traveller goes back in time and meets his grandfather as a young boy. The time traveller kills his grandfather meaning that his own parents couldn’t have been conceived, and therefore neither could he. If he doesn’t exist, he couldn’t have gone back in time to kill his grandfather, so then…he does exist after all…

Another well-known paradox is the ontological paradox which refers to the existence of an object or information where it has never been created. For example, a time traveller goes into the future and sees a beautiful painting in an art gallery. He then goes back to his own time and paints the painting he has just seen, copying from memory its design, the colours, the materials, etc.. When it is completed, the painting is considered to be so beautiful that it is put in an art gallery. That painting is the same one he saw in the future. So who created the idea of the original painting?

Such paradoxes certainly tickle the mind, and are the source of material for some excellent science fiction. After all, in fiction, anything can happen. But in real life, we’re not yet sure what would happen given scenarios such as these. We know that nature abhors a vacuum…does it abhor a paradox such as time travel?

Aside from fuzzy logic and paradoxes, there have been more scientific reasons proposed as to why the possibility of the existence of time travel remains only a pipedream. Among them:

The energy requirement for time travel is prohibitive;

Time doesn’t exist, so travelling through it can’t be possible;

Three dimensional space needs to be manipulated to allow for accelerated time;

No validation: there have been no time travellers from the past or future in our present (who have stood up to scientific questioning).

As with all subjects of lively debate, counter arguments have been presented:

Energy requirement: the phenomenal amount of energy required may well be prohibitive now, but in the future there may well be a suitable energy source with sufficient amounts of energy to allow time travel.

Time: the idea that time doesn’t exist and that it is only an abstract mathematical descriptor is by itself a statement which needs to be proved. The 11 minutes by which my wife was late when she met me at the altar were the longest of my life. There’s nothing mathematical about that! That said, if time didn’t exist but we are able to feel it and experience it, it follows that travel through this perception of time is also possible.

3D space: Einstein’s theory of relativity refers to “spacetime”, i.e. that space and time are so closely intertwined that one cannot be changed without the other. The statement that space needs to accommodate changes in time is true, but bears no bearing on the possibility of time travel.

Time travelling visitors:

They can’t time travel to us. There is an argument that backward time travel is only possible up until the moment that a means of time travel has been discovered in that past; since time travel has not yet been discovered (as such), then visitors from the future are unable to visit us.

There are, but they are disbelieved. Would you believe a person if they told you that they were a time traveller and had come to visit you from the past or the future?

There are, but they do not interact with the people or objects of the time they have travelled to (this is similar to the Star Trek Prime Directive which dictates that there can be no interference with the internal development of alien civilisations. In our time travelling scenario, the interference of a time traveller with a pre-time travelling civilisation may result in an ontological paradox over the information regarding how to time travel!) Such a directive would need to be rigorously enforced to explain the lack of witnessed time travellers.

Statistical insignificance: the chance of a time traveller travelling from some point between the start and the end of all time to our own precise position on the timeline is so infinitesimally small that it is statistically insignificant. However, that’s not to say impossible…

No interaction. There is a school of thought which argues that time travel into the past is not possible unless there is no interaction with the people and objects of the destination time. This helps to solve the ‘problem’ of time travel paradoxes.

There are other anti-time travel arguments, just as there are other counter arguments. And counter-counter arguments. The possibility (or not) of time travel is an age old question, and has attracted much attention and discussion, and I am pleased to note that this interest is increasing recently. Experiments are turning from thought experiments and what if? scenarios to scientific analyses and study which test those ideas.

For example, there is experimental evidence of time dilation (perhaps the first ‘step’ in time travel). Indeed, the atomic clocks onboard the satellites used for GPS tracking require a correction to account for the time dilation incurred from the lower gravitational potential in orbit some 20,200 km above the Earth’s surface.

A common mantra in science is “if it exists, prove it!” Such can be said, for example, of the Higgs boson particle which is theoretically possible, but as yet has remained beyond our detection. But not all things can be proven or scientifically witnessed. I can’t prove the existence of the chair I’m sitting on. I can show a picture of it, or describe it in a thousand words, tell you of its comfort, its use, its history, but it would be of no benefit. You might even sit on it with your eyes closed, but there is nothing to prove its existence. But it is there. Or consider the star I was ‘given’ as a gift. In theory it belongs to me. I have a certificate, but it proves nothing. It’s just a piece of paper with a name, a number and a location, and a note explaining that my name is entered on some database somewhere – but it doesn’t prove the existence of actual ownership.

It comes as no surprise then, that experiments and findings which have sought to prove time dilation have met with some resistance; in the above examples with the GPS atomic clocks, antagonists wager that this only shows that the clocks themselves suffer from a lower gravitational potential in orbit, and not time itself. How then to differentiate between the instrument of measurement and the quantity?

Perhaps it would be easier to prove the null hypothesis, i.e. it is easier to prove in physics that something is impossible as it lies outside an acceptable bound of a set of equations. The question we should then be asking is: why shouldn’t time travel be possible? What physics is there to forbid it?

To date, although nothing has specifically been found to show that time travel is possible, nothing has been found to prohibit it. That’s not to say that a lack of discovery is a permit – ask any teenage smoker hiding behind the bike shed! Sadly for the naughty teenager and the proponents of time travel, people are still looking for that prohibition notice (just as they are for the defining solution for the parameters of time travel).

“Is time travel possible” is a very tricky question!

Occam’s razor would state that the simplest answer to the possibility of time travel is most likely the correct one. But which is the simplest answer? No? But this immediately prompts the question of why not? (from those of us who are interested), and we immediately plunge into the details of general relativity to find out why this is the case. Far from simple. And the simple answer of yes is equally far from simple when we again need to turn to general relativity to explain why and how time travel is possible (if we best want to understand it).

Or do we? General relativity is a theory which is used to describe spacetime and it’s been well validated and proven. It is the best description of space and time that we have, and is therefore often referred to in trying to fathom whether time travel is possible. But without empirical evidence of time travel, how can we be certain that the permission or forbidding of time travel within this theory truly matches the physical environment we are in? Is the solution proving the real existence of time travel within the theory of general relativity?

Perhaps time travel can exist under certain conditions in certain solutions of the equations in general relativity, but would that really prove that time travel is possible?

Ideally, an additional theory (such as quantum theory, string theory, M-theory etc.) would support time travel given a similar set of conditions. In his book ?? Stephen Hawkins describes M-theory as a collection of theories, each of which is applicable over a specific range of conditions, but arranged such that there is an ‘overlap’ between them. It would be interesting to see if there was a time travel theory which would fit in well with M-theory.

Is time travel possible?

My head tells me not yet, though I would like to think that it is, or will be possible. Yet at the same time…I think I’m rather glad that it doesn’t yet exist – it is too dangerous. There are enough lunatics on the road who with either bad judgment or lack of brain cells cause a danger to themselves and to others. This is simple travelling in space. Who knows what would happen in the analogous case with time? And we know as well that the number of people who are able to keep time and be punctual is pitifully few. If we have no respect for a single point (or appointment) in time, how can we be trusted with travelling through it?

I see that there are two firm solutions as to whether time travel is possible – we will continue to look for a practical means of time travel, and its discovery will bear witness to the possibility of time travel. Or someone will find that physics does indeed show that time travel is impossible.

One of these outcomes will surely come…in the future. Which will be first (and accepted?) Presently, we wait…

For now (timestamp 15 August 2012 AD), we will need to be content that at least time travel certainly exists in our imagination!